With voting completed in more than half of the 50 U.S. states, the race was too close to call in Ohio, Florida, North Carolina, Pennsylvania, New Hampshire and Virginia, states that could be vital to deciding which contender wins the presidency.
Both candidates scored early victories in states where they were expected to win. Trump captured conservative states in the South and Midwest, while Clinton swept several states on the East Coast and Illinois in the Midwest.
Those victories were long predicted and not especially significant in the national race, which is likely to turn on a half-dozen toss-up states that will be crucial in the state-by-state fight for 270 Electoral College votes needed to win.
Clinton had more options to reach 270, with Trump needing a virtual sweep of about six toss-up states to win. A Clinton win in either Ohio or Florida would likely slam the door on Trump's chances. With 88 percent of the vote counted in Florida, Trump led Clinton by about 8,000 votes out of 8 million cast.
As of 8 p.m. EST (0100 GMT on Wednesday), Clinton had 68 electoral votes to Trump’s 57, with U.S. television networks projecting the winner in 15 of the 50 states.
Going into Election Day, Clinton led Trump, 44 percent to 39 percent in the last Reuters/Ipsos national tracking poll. A Reuters/Ipsos States of the Nation poll gave her a 90 percent chance of defeating Trump and becoming the first woman elected U.S. president.
Also at stake on Tuesday was control of Congress, with Republicans defending a slight four-seat majority in the 100-member Senate. The House of Representatives, where all 435 seats were up for grabs, was expected to remain in Republican hands.
Democrats scored their first breakthrough in Illinois, where Republican Senator Mark Kirk lost re-election. But Republicans Rob Portman in Ohio and Marco Rubio in Florida won high-profile Senate re-election fights.
In a presidential campaign that focused more on the character of the candidates than on policy, Clinton, 69, a former U.S. secretary of state, and Trump, 70, a New York businessman, accused each other of being fundamentally unfit to lead the country.
Trump again raised the possibility on Tuesday of not accepting the election's outcome, saying he had seen reports of voting irregularities. He gave few details and Reuters could not immediately verify the existence of such problems.
In North Carolina, the state elections board extended voting hours in eight Durham County locations after technical errors led to long waits.
Financial markets, betting exchanges and online trading platforms largely predicted a Clinton win, although Trump's team said he could pull off a surprise victory like the June "Brexit" vote to pull Britain out of the European Union.
The economy, terrorism and healthcare ranked as the top three concerns facing Americans casting ballots in Tuesday’s election, according to an early reading from the Reuters/Ipsos Election Day poll.
The poll of about 35,000 people found that 25 percent of voters picked the economy as the “most important problem.”. Another 14 percent named “terrorism/terrorist attacks” and 13 percent picked healthcare.
A signature Trump issue, immigration, was chosen by 7 percent of voters as the most important issue in Tuesday's poll.
Some 15 percent of Americans who cast a ballot on Tuesday said it was their first time voting in a presidential election, up from 9 percent in 2012, the poll showed. Thirteen percent of voters waited until the final week to make up their minds.
ASIAN MARKETS ON TENTERHOOKS
The Dow Jones Industrial Average index ended up 0.4 percent on Tuesday as investors bet on a win for Clinton, seen by Wall Street as more likely to ensure financial and political stability. Mexico's peso whipsawed in after-session trading on Tuesday, but remained at two-and-a-half-month highs on expectations of a defeat for Trump, who has threatened to rip up a free-trade deal with Mexico.
In Asia, markets were on tenterhooks at the start of the day on Wednesday as early results showed wins for both candidates and no clear trend as yet, causing a nervous shift back to safe haven assets.
Majorities of American voters told pollsters they viewed both candidates unfavorably after a particularly bruising and divisive campaign that began in early 2015. For months, polls showed both Clinton and Trump were unpopular, although Trump was more so.
In Portsmouth, Ohio, Vada Royster, 44, said she voted for Clinton based on her experience for the job. "I chose the lesser of two evils," she said.
Royster’s boyfriend, Steve Conley, also said he voted for Clinton even though he did not think a woman should be president.
"I wouldn't trust Trump with the nuclear codes," Conley said. "He's a hothead. I'd be afraid he was going to try to show his power, to prove his power."
Trump, who has never previously held political office, pledged to shake up the Washington establishment but also alienated many people, including in his own party, with a campaign heavy on personal insults and unorthodox positions such as a proposal to build a wall along the Mexican border to keep out illegal immigrants.